Alumina body having nano-sized open-cell pores that are stable at high temperatures

ABSTRACT

An alumina body having nano-sized open-cell pores, the alumina body is formed from α-Al 2 O 3  and Al(OH) 3 . The alumina body has porosity of greater than 36-percent by volume and a mean pore flow diameter less than 25-nm. The alumina body retains porosity of over 20-volume percent for temperatures up to 1510° C. for 1-hour. The nano-sized open-cell porous body can be scaled to any 3-dimensional structure.

RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.15/409,619, filed on Jan. 19, 2017, which claims the benefit of priorityof U.S. Provisional Patent Application No. 62/289,149, filed Jan. 29,2016, which is herein incorporated by reference.

FIELD

The present invention generally relates to an alumina body havingnano-sized open-cell pores. More specifically, the alumina body has openpores with mean pore flow diameters less than 25-nm and that retainporosities of at least 20-percent by volume for temperatures of up to1510° C.

BACKGROUND

Open-cell porous bodies can be used as filters in a variety ofapplications. Very fine porosity is desired for chemical processing,pharmaceutical processing, refining waste water, purifying foods andenergy production to name a few. The filters employed in these processesare used to purify, concentrate, sterilize and separate materials. Thelisted applications require filters with pores sizes in theultrafiltration (100 nm to 10 nm) and nanofiltration (10 nm to 1 nm)ranges. Some applications require the filters survive thermal excursionsin excess of 1500° C., such as in high temperature gas mixing. Typicallyfilters having pores in the low end of ultrafiltration and into thenanofiltration range have required the use of thin membrane (e.g.polymers) of nano-sized pores supported on a substrate (e.g. metal,polymer, ceramic) having larger pores. In general filters in thenanofiltration range are complex to manufacture, do not sustainsignificant amounts of open porosity during high thermal excursions andare difficult to reproduce in 3-dimensional structures.

U.S. Pat. No. 6,565,825 to Ohji, which is herein incorporated byreference, has shown that alumina powders can be sintered to form porousalumina structures. However, sintering temperatures in excess of 1250°C. reduce porosities to 36-volume percent or below. Ohji further showsthat combining alumina hydroxide Al(OH)₃ with the alumina powder, andthen subsequently sintering, can transform the Al(OH)₃ through γ→θ→αphases to provide materials that maintain porosities of 36-volumepercent up to 1250° C.

The present invention aims to eliminate the need for a membranesupported by a substrate and provide nano-sized open-cell porosity thatcan be scaled to any 3-dimensional structure. The present invention alsoaims to improve the thermal stability of highly porous materials tobeyond 1500° C.

SUMMARY

The present disclosure is directed to a ceramic body, the ceramic bodycomprising α-Al₂O₃ having a porosity greater than 36-percent by volume,a mean pore flow diameter less than 25-nanometers, and a porosity thatstays above 20-percent by volume at an annealing temperature of 1510° C.for 1-hour.

Another aspect of the present patent application is directed to a methodof fabricating a ceramic body, comprising the steps of providingcrystalline α-Al₂O₃ particles having D50 of 0.4-0.6 microns and Gibbsitephase Al(OH)₃ particles of D50 of 5-6 microns. The method then involvescombining, milling and granulating the particles. The method furtherinvolves forming a green compact and sintering the green compact in atemperature range of 1316° C. to 1510° C.

BRIEF DESCRIPTION OF DRAWINGS

For the purposes of illustrating the invention, the drawings showaspects of one or more embodiments of the invention. However, it shouldbe understood that the present invention is not limited to the precisearrangements and instrumentalities shown in the drawings, wherein:

FIG. 1 is a schematic, sectional view of one exemplary embodiment of theceramic body according to the present invention;

FIG. 2 is a schematic diagram illustrating open pore sizes achievable bythe current invention with exemplary filtration ranges and attributes;

FIG. 3 is DSC-TGA plot for the alumina trihydrate used in the presentinvention;

FIG. 4 is a plot of volume percent porosity versus sintering temperaturefor the ceramic bodies of the present invention;

FIG. 5 is a plot of mean pore diameter versus sintering temperature forthe ceramic bodies of the present invention;

FIG. 6 is a plot of porosity versus time for a ceramic body of thepresent invention at 1510° C.; and

FIG. 7 is a schematic diagram for thermodynamic phases and stabletransitions of alumina.

DETAILED DESCRIPTION

FIG. 1 shows ceramic body 20. Ceramic body 20 is a 3-dimensional ceramicbody or a thin layer polished from such a body. Ceramic body 20comprises α-Al₂O₃ having nano-sized pores 22. It is critical to have avolume porosity greater than 36-percent so that the alumina body has asignificant number of open pores that allow fluids to flow therethrough. The mean pore flow diameter within porous body 20 should beless than 25-nanometers. It is critical to have this mean flow porediameters less than 25-nanometers in order to perform the processes aslisted in FIG. 2.

Raw materials used in the preparation of the examples (samples A-E)described below are crystalline alpha-alumina (α-Al₂O₃) and aluminatrihydrate (Al(OH)₃) in the ratio of 40-percent hydrated alumina to60-percent alpha-alumina by weight. The hydrated alumina had a syntheticGibbsite phase structure as verified by the DSC-TGA curves in FIG. 3. Itwas found that the 40-percent hydrated alumina to 60-percentalpha-alumina ratio was ideal for achieving good porosity though othercompositions where α-Al₂O₃ particles in the range of 50-90 percent couldbe used. The α-Al₂O₃ crystalline particles used in the preparation ofthe examples described below are Pechiney powders, specifically PechineyP172 SB03 having a D50 particle size of 0.4-0.6 microns. Hydratedalumina particles used in the preparation of the examples below wereJ.M. Huber Corporation powders, specifically HYDRAL® 710 with D50particle size of 5.0-6.0 microns. One or more organic binders incombination with water, a dispersant, and a lubricating agent were mixedwith all particles to form a slurry. The organic binders act as abinding agent that holds the mixture of particles together. Duringsintering the organic binders burn off leaving the shape of the bodyintact. Some examples of organic binders that may be used to form greencompact include polyvinyl alcohol (PVA) and polyethylene glycol (PEG).Other binders may include, but are not limited to, acrylic binders, gumsand waxes.

Table 1 lists the formulation used to produce the porous alumina samplesanalyzed.

TABLE 1 Exemplary Formulation for Al₂O₃ (weight percent) Material Weight(kg) Manufacturer Alpha Phase Alumina Al₂O₃ 90.0 Pechiney HydratedAlumina Al(OH)₃ 60.0 J. M. Huber Corp. Dispersant 1.5 Organic Binder I8.8 Organic Binder II 4.2 Lubricating Agent 2.5 Water 55.5

General preparation of the new porous alumina formulation is as follows.Water is placed in a tank and mixed under a high shear mixer. The pHlevel is adjusted to between 8.8 and 9.5. The dispersant is then addedto the mixture. After the solution is adequately mixed the solution ispoured into a ball mill and a measured amount of hydrated aluminaHYDRAL® 710 is added. After the hydrated alumina is adequately mixed thealpha alumina is added and the slurry is subsequently milled for2-hours. Organic binder I, organic binder II, and the lubricating agentare then added and milled for an additional 1-hour. The resulting slurryis spray dried into granulated powder and then pressed into a greencompact of a given shape. The green compact is then heated to temps of300° C. to 600° C. as part of a binder burnout cycle. The compact isthen further heated to a sintering temperature of 1316° C. to 1510° C.with a 1-hour soak time. The sintering temperature helps determine theporous properties of the material, with higher temperatures trendingtoward less porosity and larger maximum pore sizes. The firing ranges ofthe porous alumina samples are listed in TABLE 2. TABLE 2 additionallylists process parameters along with mechanical and porous properties.

TABLE 2 Formulations and Properties of Al₂O₃ Porous Substrates SampleSample Sample Sample Sample A B C D E Sintered Temp 1316 1343 1399 14541510 (° C.) Time (hours) 1 1 1 1 1 Percent Porosity 46.4 42.9 37.1 28.322.3 Mean Flow 0.0208 0.0219 0.0200 Pore Diameter (microns) Bubble Point0.0752 0.0837 0.0606 Pore Diameter (microns) Std. Dev. of 0.0177 0.01430.0137 Avg. Pore Diameter (microns) Diameter at 0.0137 0.0137 0.0169 MaxPore Size Dist. (microns) Bulk Density 2.126 2.24 2.496 2.849 3.091(g/cc)

TABLE 2 compares various properties of the differently sintered samples.All samples A-E were made from the formulation in TABLE 1, but kilntemperatures were altered to vary the mechanical and porous properties.Firing temperatures ranged from 1316° C. to 1510° C. Percent porosity byvolume ranged from 46.4-percent (1316° C.) to 22.3-percent (1510° C.).Samples sintered at temperatures roughly 1400° C. or lower hadporosities greater than 36-percent by volume. Mean flow pore diameterremained steady in a range of 0.0200-0.0219 microns, FIG. 5, while thebubble point, or largest pore size, ranged from 0.0606-0.0752 microns.The diameter at max pore size distribution, which indicates the mode ofthe pore sizes, ranged from 0.0137-0.0169 microns, and the standarddeviation of the pore sizes ranged from 0.0137-microns to0.0177-microns. All pore size distribution and permeability data wastaken using capillary flow porometry. Specifically, the machine used wasa model CFP-1500AEM Capillary Flow Porometer produced by PorousMaterials, Inc. It should be noted that the pore size distribution andpermeability data in TABLE 2 is specific to this machine and to thetesting parameters applied during the tests. It should be noted that dueto limitations of this machine, open-cell pore sizes are measurable downto only 13-nm. This does not preclude the existence of pores less than13-nm, but these pores are not measurable with the equipment being used.

A plot of percent porosity versus sintering temperature, FIG. 4, showsthat the percent porosity decreases with increasing temperature, butstays above 36-percent by volume for temperatures of 1316° C. to 1400°C. The percent porosity also stays above 20-percent by volume fortemperatures under 1510° C. FIG. 6 shows that at 1510° C. increasedannealing time beyond 1-hour decreases percent porosity to about16-percent by volume after 3-hours.

The present data shows that ceramic body 20 composed of porous α-Al₂O₃has porosities of 36-volume percent or greater after annealing for1-hour at temperatures up to 1400° C. This is 150° C. greater than priorart materials utilizing Al(OH)₃. This unexpected result is believed tobe a result of a combination of larger initial particle sizes and thesynthetic Gibbsite structure of the Al(OH)₃ particles. Differentthermodynamic phase transitions, FIG. 7, are believed to be contributingto the higher percent porosity and higher thermal stability of thepores. Ceramic body 20 can therefore have a final porosity greater than20-percent by volume when annealed at a temperature over 1350° C. for1-hour. The new structure provides a new, higher temperature resistantporous alumina body that can be used for nanofiltration and that can beshaped into any 3-dimensional structure.

While several embodiments of the invention, together with modificationsthereof, have been described in detail herein and illustrated by theaccompanying examples, it will be evident that various compositions andfurther modifications are possible without departing from the scope ofthe invention. The scope of the claims should not be limited by thepreferred embodiments set forth in the examples, but should be given thebroadest interpretation consistent with the description as a whole.

1. A ceramic body, comprising: α-Al₂O₃ having a porosity of greater than36 percent by volume and a mean open pore flow diameter less than 25nanometers.
 2. A ceramic body as recited in claim 1, wherein saidporosity stays above 20 percent by volume at an annealing temperature of1510° C. for 1 hour.